Antimicrobial resistance is a major public health problem worldwide. Neisseria gonorrhoeae (Ng), the causative agent of the sexually transmitted infection gonorrhea, has become multidrug-resistant and has achieved ?superbug? status. In addition, between 6% and 12% of women successfully treated for gonorrhea are re-infected within three months. Novel therapeutics against Ng are urgently needed. Complement (C?) is a key arm of innate immune defenses. A mechanism used by several pathogens, including Ng, to escape C? is to bind to a host C? inhibitor called factor H (FH). FH comprises 20 domains, arranged in an extended head-to-tail fashion. Only the four N-terminal domains (domains 1-4) possess C? inhibiting activity; the remainder of the molecule is important for recognition of host surfaces. Many pathogens, including Ng, have evolved to bind FH through domains 6-7 and/or 18-20. A recombinant fusion of FH domains 18-20 (with a point mutation in domain 19 to abrogate binding to host cells) to IgG Fc (FH*/Fc) binds to and promotes C?-dependent killing of Ng. Topically administered FH*/Fc attenuates Ng infection in the mouse vaginal colonization model. We have produced, in our plant expression system, variant FH*/Fc molecules with different Fc or different linkers between FH and Fc. We demonstrated the functional superiority of plant-made FH*/Fc variants incorporating flexible linkers, (GGGGS)2 or (GGGGS)3, both in vitro and in a mouse vaginal infection prophylactic model. We have also shown that the functionality of these molecules depends on the ability of the Fc to activate complement on the Ng surface. We envision using FH*/GS-hFc to prevent re-infection in women treated for uncomplicated gonorrhea. In this Fast-Track project we seek to further preclinical development of this promising immunotherapeutic against drug-resistant Ng. In Phase I we will produce and test in vitro five new FH*/GS-hFc variants where the Fc is modified to improve C-mediated killing of Ng and identify two lead variants with the greatest potency. In Phase II we will compare the potency of these FH*/GS-hFc lead variants in vivo against four divergent Ng isolates, determining the minimum effective dose. We will test their in vitro potency against 50 diverse Ng clinical isolates. We will scale up purification and evaluate the ability of the two lead variants to undergo spray-drying and retain in vitro potency. Based on the sum of all the above experiments, we will select one variant as a lead for commercialization. We will formulate the lead FH*/GS-hFc variant in an intravaginal ring designed for sustained, controlled release over several weeks, and evaluate its PK and safety in rhesus macaques. We will perform a six-month drug substance stability study in anticipation of a future Phase 1 clinical trial. With the help of a large contract manufacturer of plant-made proteins, we will conduct a technoeconomic analysis to determine the commercial viability of plant-made FH*/GS-hFc, and seek the guidance of the FDA on future FH*/GS-hFc non- clinical and clinical development.